Filtration of alkaline waters



Oct. 9, 1928. 1,686,558

R. E. HALL FILTRATION OF ALKALINE WATERS Filed May 5, 1924 INVENTOR Patented oer. e, 1928.

UNITED STATES 1,686,558 PATENT OFFICE.

mrrrn. HALL, or rrrirsnuaen, PENNSYLVANIA, assrenoa 'ro JOHN M. norwoon, I or noamonr BOROUGH, rnnnsnvanm.

FILTRATION OF ALKALINE WATERS.

Application filed my "a,

i The present invention relates to the filtration of alkaline waters, and more especially to the filtration of the hot alkaline waters encountered in modern steam boiler ractice. In conditioning water for steam boi ers, and particularly in treating steam boiler water to soften it, alkaline chemicals, such as soda Such treated boiler water is'distinctly a ka- I ash or lime or both, are commonly emplo ed.

line, the alkalinity frequently being equivalent to twenty to thirty milliequivalents of sodium carbonate.

. In order to remove the sludge which forms in the boiler, due to the concentration of solids incident to evaporation and conditioning, a portion of the water is frequently drawn from the boiler and passed through a lation of a portion of the boiler water through a filtering device to remove suspended solids is shown, for example, in my copending application Serial No.- 692,804.

The usual water filtering material is sand. Sand consists principally of silica (S1O While sand ispa satisfactory filterlng material for ordinary waters, or even for cold alkaline waters, it is found that the alkaline water which is passed through a filter at the temperatures encountered in modern steam boiler practice dissolves the sand. The rate of solution of the sand or silica s rather slow, but is suflicient to be perceptible 1 n a relatively short "time on a filter in'contmuous service, filtering the hot alkaline boiler water. Moreover, the silica which is dissolved asses into the boiler as an added im-' purity 1n the water.

In the drawin s I have indicated the usual section, through a steam bo1ler,'showing diagrammatically the connection of the filter t the boiler, and

Figure 2 is a detail vertical section through the filter.

Referring to the drawings the reference 1924. Serial No. 710,740,

numeral 1 indicates a steam boiler which is supplied with treated water. A quantity of boller water, say -an amount equivalent to say 5 or of the feed water, is continuously circulated through a sludge removnig filter 2, being withdrawn from the boiler through the pipe 3 and re-introduced into the boiler through-the pipe 4." A pump 5 maintains a continuous circulation through the filter. The boiler water is treated pref erably with sodium carbonate, although any of the other usual alkaline treatments may boiler with the feed water through the feed water pipe 10, or may be introduced at any desired place in'the boiler. through an'entirely separate p pe, such as.shown, for example, at 11. The 'filter'2 is shown in more detail in Figure 2. It consists of a casing holding a bed of filtering material 12 through which the boiler water is assed for the removal of the suspended slu ge, whichusually consists principally of calcium carbonate. wardly through thefilter bed during the filtering operation. In order to wash the filtering material, the filter is periodically shut off from the boiler and-washout water is forced upwardly through the filter bed by means of washout pipes 13 and 14. Amanually operable agitating device 15 is pro-' vided for stirring the filter material during the washout. I

Silica sand has heretofore been usually employed as the material for the filter bed 12, but I replace it with a material which is substantially insoluble as compared. with the silica sand under the conditions of alkalinity and heat encountered in steam boiler service. Instead of using a material like silica which is an acid anhydride, I employ a material in which the negative or acidradical is neutralized by a basic or positive radical,

The hot boiler water passes down-- so that the material does not react like the silica-with the hot alkalineboiler water. I prefer to use a silicate having as its positive or basic radical an element with which the ture is increased to the points encountered silica ($0,)

boiler water is ,fully saturated, such, for ex ample, as iron, magnesium or calclum, although other basic radicals may be employed to neutralize the acid radical, which, if exlstanhyride. contact with line substance, say, hydroxide, the condition may .by the following chemical equation:

s10 sio (So1ld) (dissolved) I mo 4211.240. 2 211+ SlOrzmon 2OH-+ 2Na 21110 NagSlOa Atordinary room temperatures, the equation proceeds from left tori ht so slowly that the loss of a solid silica lter material is negligible. However, when the temperainboiler practice, the rate' of solution and hydration of the silica is sufficiently great to be no longernegligible.

If, in place of an acid anhydride, 9. substance is selected for the filtering medium 35 in which the acidic characteris already neutralized by a basic radical, and the solubility of which in pure water is low, of solution inthe hot alkaline boiler "water much less. As a typical example, we may consider the chemical reactionsln case an iron silicate (Fe SiOQ is employed. The followiii chemicalformula shows the type of equili rium encountered: asses maon 12 4011- u anion), Name.

If we follow through the equilibrium irom' left to right,

we note that by increasing the amount of hydroxyl ion in the solution, we will decrease the amount of iron ion by the formation of undissociated iron hydroxide,

, and thereby accelerate solution, and that by increasing the. amount of sodium :ion, we

tend to form more of the sodium ortho-silicate molecules' However,

the ionization of the iron hydroxide, even under these conditiohs, aslcompa'r d to that of water, is very large, so that a relatively large amount of iron ion will exist in the solution. Also at equivalent concentrations, the ionization of asalt containing a univalent ion, such as sodium, isalways markedly higher than that ofa then the rate the sludge.

salt witha polyvalent ion,- such as iron,-

consequently the tendency for the iron ion A silicate-of iron to be removed from their nfluence in the equilibrium with..the iron silis' cate by being. transformed. into undisso'ci ated iron hydrate and sodium ortho-silicate is in no wise so large as that sponding ions in equation (1)- indicating the equilibrium condition for silica. Moreover, in usual boiler water treatment, the boiler water is fully saturated with respect to iron. A similar equilibrium cate, such, for example, (Mg SiO,), belonging to the olivine group of minerals, is indicated 'by the following equation:

M95104 M 18104 2Mg +Si0r--- (Solid) dissolved) 4NaOH a 4011 4Na As in the case of iron, the hydrate of magnesium has a very lar e ionization as compared with water, so t at a relatively large amount of magnesium ion willexist in the solution. Moreover, in the usual boiler water. treatment, the water is fully saturated with respect to magnesium.

A similar equation might be drawn for a calcium silicate showing that the chemical reactions are such that it will be. dissolved in hot alkaline solutions ver much more slow- 1y than the acid anhydri e silica.

In addition to being very diflicultly soluble in hot alkaline boiler water, the material should be capable of being crushed into a sand of gram size suitable for filterin Also the material should have a relative y high s 'ecific gravity to allow a rapid washout. he ater. the specific gravity, the more rapi y' can washout water'be passed upwardly through for a magnesium silias ,forsterite for the correthe filter bed to wash out The preierred material for the filter bed I I is a granulated smelter slag obtained from copper smelting and havingappro'ximately' the following composition:

Per cent.

F80 43. 24 SiO 38.77 Al,(), 9.46 g0 3.01 CaO '1. 72 BaO .25 ZnO I .63 ZnS-.. v 2. 01 CnO .48 PhD. 15 AS .09 Ni .04 As shown by the formula, this consists principally ofv silica which is substantially.

neutralized by the basic iron, the silica prob,-

ably existing in the form of an iron silicate.

The specific gravity ofthe material is about no. I

3. L Its rate of solution in the alkaline waters at boiler temperatures is a proximately 1/35 of that for silica sand An other suitable material is crushed forsterite which is magnesium silicate '(Mg SiO Forsterite is one of the olivine grou of 'minerals. It has a specific gravity ofa out 3 and a solubility in alkaline waters at boiler temperatures of very much less than that of Other members of the olivine group may be used, and also some members of the pyroxene and amphibole groups of minerals may be used. These have specific gravities of about 3 or over, and are very difiicultly soluble in the hot alkaline boiler water. While the negative radical in the minerals used for the filter bed will usually be a silicate or contain a silicate, because of their more ready availability, other minerals may be used in which the negative radical may be other than silicate, such, for exam le, as ihnenite (FeTiO or chromite (FeC Ir OQ. Chromite, for example, has a specific gravity of over 4 and its rate of solution in the hot alkaline water is approximately only 1/23 of that of silica.

ter slag While I have set forth the best materials known to me as the filtering material, it is to be understood that the invention is not limited to these specific materials, but may be otherwise embodied within the scope of the invention as defined in the broader claims. 7

I claim: 1. As a filtering medium for hot alkaline boiler waters, an iron silicate.

2. As a filtering medium for hot alkaline boiler waters, granulated smelter slag consistin principally of an iron silicate.

3. 5s a filtering medium for hot alkaline waters, granulated smelter slag consisting principally of an iron silicate and having a specific gravity of over 3.,

4. The process of filtering hot alkaline boiler waters, which consists in passing the heated boiler water through a filter bed consisting principally of a granulated smelcomposed principally of an iron silicate.

In testimony whereof I have hereunto set my hand.

RALPH E. HALL. 

